1. Field of the Invention
The present invention generally relates to an integrated optical device having at least two optical devices integrated therein and a method of fabricating the integrated optical device, and in particular, to an integrated optical device in which optical devices are easily isolated electrically from each other and a method of fabricating the integrated optical device.
2. Description of the Related Art
For ultra high-speed optical signal transmission at 10 Gbps or higher, a wavelength band around a central wavelength 1550 nm is usually adopted as it experiences less optical loss in the fiber. An electro-absorption optical modulator is typically used in combination with a DFB LD (Distributed-Feedback Laser Diode) to fabricate an optical transmission system. However, in the case of a laser combined with the electro-absorption optical modulator, that is, an electro-absorption modulated laser (EML) is used, it is not effective in the long-distance transmission due to its low optical output. Therefore, to increase optical output, integration of an SOA (Semiconductor Optical Amplifier) has been proposed.
In an integrated device, electrical isolation between a DFB LD and an electro-absorption optical modulator (or electrical isolation) and optical isolation between an SOA and the electro-absorption optical modulator or between the SOA and the DFB LD is a significant factor which determines the level of performance.
FIG. 1 illustrates the structure of a conventional PBH (Planarized Buried Hetero-structure) integrated optical device, and FIG. 2 illustrates the structure of an EMBH (Etched Mesa Buried Hetero-structure) integrated optical device.
Referring to FIG. 1, a PBH integrated optical device 100 comprises a first optical device 110, a second optical device 120, and an electrical isolation area 130 provided therebetween Material layers of the optical devices 110 and 120 are formed on a substrate 101 and a light waveguide 102. The light waveguide 102 is preferably surrounded by a clad layer 103. An electrode for flowing current is provided in each of the first and second optical devices 110 and 120.
To fabricate the PBH integrated optical device 100, an active layer and the p-clad layer 103 are grown on the n-substrate 101. After the light waveguide 102 is formed into the shape of a mesa by etching the active layer and the p-clad layer 103, an SI (Semi-Insulating) clad layer 104 is formed around the light waveguide 102, and a p+-clad layer 105 is grown on the overall substrate 101. Subsequently, the electrical isolation area 130 is formed between the first and second optical devices 110 and 120 over the light waveguide 102 for allowing the light to pass between the optical devices 110 and 120, thus electrically isolating the optical devices from each other. The formation of the electrical isolation area 130 is typically carried out by changing the property of the p+-clad layer 105 through ion implantation, or by etching the p+-clad layer 105.
The ion implantation has some drawbacks. First, it is accompanied by one or more photolithography processes which tend to provide a semiconductor crystalline structure with injected ions. Also, the manufacturing equipments required in the ion plantation are expensive. The etching requires photolithography, and excessive etching tend to damage the light waveguide and yields unstable throughput. Furthermore, since the p+-clad layer is formed over a wide area, an additional etching is needed to reduce the resulting electrostatic capacity.
Referring to FIG. 2, an EMBH integrated optical device 200 comprises a first optical device 210, a second optical device 220, and an electrical isolation area 230 between them. To fabricate the EMBH integrated optical device 200, an active layer, a p-clad layer 203, and a p+-clad layer 204 are formed on an n-substrate 201. A light waveguide 202 is formed in the shape of a mesa by etching them. An SI clad layer 205 is filled around the light waveguide 202. Since the p+-clad layer 203 is positioned over the light waveguide 202, the EMBH integrated optical device has a reduced electrostatic capacity, which obviates the need for etching an unnecessary portion of the p+-clad layer.
Similar to the PBH integrated optical device, the optical devices 200 are connected by the p+-clad layer in the EMBH integrated optical device. Therefore, they need to be electrically isolated from each other by ion implantation or etching with shortcomings inherent to the ion implantation or etching. Moreover, a narrow width of the p+-clad layer for flowing the current therethrough increases resistance including a contact resistance between the p+-clad layer and an electrode and, as a result, renders the electrode to be sharp enough to cut the electrode in some cases.